Recently there has been considerable interest within the detergents industry in the production of detergent powders having a relatively high bulk density, for example 600 g/l and above.
Generally speaking, there are two main types of processes by which detergent powders can be prepared. The first type of process involves spray-drying an aqueous detergent slurry in a spray-drying tower. In the second type of process, the various components are dry-mixed and optionally agglomerated with liquids, e.g. nonionics.
The most important factor which governs the bulk density of a detergent powder is the bulk density of the starting materials in the case of a dry-mixing process, or the chemical composition of the slurry in the case of a spray-drying process. Both factors can only be varied within a limited range. For example, one can increase the bulk density of a dry-mixed powder by increasing its content of relatively dense sodium sulphate, but the latter does not contribute to the detergency of the powder, so that its overall properties as a washing powder will generally be adversely affected.
Therefore, a substantial increase in bulk density can only be achieved by additional processing steps which lead to densification of the detergent powders. There are several processes known in the art leading to such densification. Particular attention has thereby been paid to the densification of spray-dried powders by post-tower treatment.
In our co-pending European patent application 89202706.1 a process for the continuous preparation of a granular detergent composition or component having a bulk density of at least 650 g/l is described. This process comprises the steps of treating a particulate starting material
(i) in a first step in a high-speed mixer/densifier, the mean residence time being from about 5-30 seconds; PA0 (ii) in a second step in a moderate-speed granulator/densifier, whereby it is brought into, or maintained in, a deformable state, the mean residence time being from about 1-10 minutes and PA0 (iii) in a final step in a drying and/or cooling apparatus. PA0 (i) in a first step in a high-speed mixer/densifier, the mean residence time being from about 5-30 seconds; PA0 (ii) in a second step in a moderate-speed granulator/densifier, whereby it is brought into, or maintained in, a deformable state, the mean residence time being from about 1-10 minutes and PA0 (iii) in a final step in a drying and/or cooling apparatus,
Preferably, the particulate starting material is already brought into, or maintained in, a deformable state in the first step.
The advantages of the process described above are the fact that it can be carried out in a continuous way and that it is relatively flexible with respect to the composition of the starting materials.
Essential for the process is the deformable state--to be defined hereafter--into which the particulate starting material must be brought. This can be induced in a number of ways, for instance by operating at temperatures above 45.degree. C. and/or adding liquids to the particulate starting material.
Optimal densification results are obtained when the starting material is very deformable. However, when processing very deformable powders, complications may arise with regard to the particle size distribution of the final product. More in particular, a considerable production of oversize particles was observed. This was found to be especially the case when using starting materials which have a high active content, i.e. a content of anionic and/or nonionic surfactants of 20% by weight or more of the starting material.
It is therefore an object of the present invention to provide an improved continuous process of the above-mentioned kind for obtaining high bulk density detergent compositions, or components thereof, having a bulk density of at least 650 g/l.
We have now found that a substantial improvement with regard to the particle size distribution can be achieved when 0.1 to 40% by weight of a powder is added in the second step or between the first and the second step. In carrying out this improved process, the particle porosity, which may be in the order of 20-70% for a spray-dried base powder, is successfully reduced to, or kept at, values of less than 10%, preferably less than 5%. At the same time, the production of "oversize" particles is kept within acceptable limits.
JP-A-63/099296 (LION) discloses a process for manufacturing a granular detergent material wherein the formation of fine powder and coarse grains is suppressed by spraying 7-15% by weight of water and nonionic surfactant as a liquid binder system on to the base powder, followed by admixing 10-25% by weight of a water-insoluble, fine powder, such as a zeolite, in a granulator.